Eddy covariance flux measurements of gaseous elemental mercury over a grassland

Abstract

Direct measurements of the net ecosystem exchange (NEE) of gaseous elemental mercury (Hg 0 ) are important to improve our understanding of global Hg cycling and, ultimately, human and wildlife Hg exposure. The lack of long-term, ecosystem-scale measurements causes large uncertainties in Hg 0 flux estimates. It currently remains unclear whether terrestrial ecosystems are net sinks or sources of atmospheric Hg 0 . Here, we show a detailed validation of direct Hg 0 flux measurements based on the eddy covariance technique (Eddy Mercury) using a Lumex RA-915 AM mercury monitor. The flux detection limit derived from a zero-flux experiment in the laboratory was 0.22 ng m −2 h −1 (maximum) with a 50 % cutoff at 0.074 ng m −2 h −1 . We present eddy covariance NEE measurements of Hg 0 over a low-Hg soil (41-75 ng Hg g −1 in the topsoil, referring to a depth of 0-10 cm), conducted in summer 2018 at a managed grassland at the Swiss FluxNet site in Chamau, Switzerland (CH-Cha). The statistical estimate of the Hg 0 flux detection limit under outdoor conditions at the site was 5.9 ng m −2 h −1 (50 % cutoff). We measured a net summertime emission over a period of 34 d with a median Hg 0 flux of 2.5 ng m −2 h −1 (with a −0.6 to 7.4 ng m −2 h −1 range between the 25th and 75th percentiles). We observed a distinct diel cycle with higher median daytime fluxes (8.4 ng m −2 h −1 ) than nighttime fluxes (1.0 ng m −2 h −1 ). Drought stress during the measurement campaign in summer 2018 induced partial stomata closure of vegetation. Partial stomata closure led to a midday depression in CO 2 uptake, which did not recover during the afternoon. The median CO 2 flux was only 24 % of the median CO 2 flux measured during the same period in the previous year (2017). We suggest that partial stomata closure also dampened Hg 0 uptake by vegetation, resulting in a NEE of Hg 0 that was dominated by soil emission. Finally, we provide suggestions to further improve the precision and handling of the "Eddy Mercury" system in order to assure its suitability for long-term NEE measurements of Hg 0 over natural background surfaces with low soil Hg concentrations (< 100 ng g −1 ). With these improvements, Eddy Mercury has the potential to be integrated into global networks of micrometeorological tower sites (FluxNet) and to provide the longterm observations on terrestrial atmosphere Hg 0 exchange necessary to validate regional and global mercury models.

Seasonal gaseous elemental mercury fluxes at a terrestrial background site in south-eastern Australia

“…Reduced CO 2 uptake is evident by the slight dip in CO 2 diel flux compared to the stronger diel trends observed in other seasons. Osterwalder et al (2020), observed a similar phenomenon where drought conditions at a grassland site in Switzerland resulted in reduced CO 2 and Hg stomatal uptake. As intense drought conditions are a defining feature of Australia's environment it is plausible that during drought Australia's grasslands could become a greater source of atmospheric Hg.…”

Critical Observations of Gaseous Elemental Mercury Air‐Sea Exchange

“…However, before we are able to discuss consequences of using a cubic wind speed parameterizations of k Hg in detail, we need more long‐term, direct Hg 0 flux measurements, across a greater range of wind speeds. The recent presentation of the first eddy covariance system with sufficient accuracy to measure Hg 0 land‐atmosphere exchange over natural surfaces (Osterwalder et al., 2020) might be more easily applied to measure air‐sea Hg 0 exchange than the more technically challenging REA method used in this study. Before applying this novel eddy covariance method for Hg over oceans, though, the accuracy will need to be improved compared to what was achieved in the premier application of this method, given the generally lower gradients over water than land.…”

“…We need multi‐seasonal Hg 0 flux time series to make statistically significant estimates of the dependence of fluxes on other environmental parameters, including complementary observations relevant to hypothesized explanations such as the role of sea spray aerosol emission, occurrence of biological surface films or bubble formation. This can be done with direct Hg 0 flux measurements on islands, floating instrument platforms or research vessels using the newly available eddy covariance technique (Osterwalder et al., 2020). If there is indeed a stronger cubic wind speed dependence for k Hg , similar to that of O 2 which also has low solubility, there will be a need to reevaluate the understanding and modeling of Hg 0 flux processes at local, regional and global scales.…”

Automatic monitors for direct continuous mercury measurement in ambient air, hydrocarbon, and industrial gases

“…Unique features of the monitor, which provided direct real-time measurement with frequency of 15 times per second readings, were used for the specific mercury surface to air mercury flux calculation (Osterwalder et al, 2020).The monitor can also be used in applications when the analysed ambient air has a reduced pressure, down to 40 kPa (at an altitude up to 5300 m or in the plane measurements (Weigelt et al, 2016)). Since the monitor does not require consumables, including gas cylinders, the use of RA-915AM monitors makes it possible to simplify maintenance at high-altitude and remote monitoring stations.…”